1. Field of the Invention
This invention relates to a method for starting up a reactor and a reactor system. More particularly, this invention relates to a method for starting up a reactor and a reactor system in producing (meth)acrylic acid and/or (meth)acrolein by the reaction of catalytic gas phase oxidation.
2. Description of Related Art
Acrylic acid is used as coating materials, fiber processing materials, leather processing materials, and building materials in addition to being adopted as a copolymer for acrylic fibers or in the form of emulsion for an adhesive agent. It has been filling a growing demand from the applications concerned. Thus, the desirability of developing a process which produces the acrylic acid from less expensive raw materials, allows expansion of the plant operated for the production, and entails environmental pollution only sparingly has been finding enthusiastic approval. The acrylic acid is generally produced by the reaction of catalytic gas phase oxidation of propylene, for example.
This reaction of catalytic gas phase oxidation for the production of acrylic acid, for example, is exothermal in nature. The maintenance of this exothermal reaction at a constant temperature is generally accomplished by circulating a heat medium with a pump to cool the reaction tube and then cooling the heat medium with a cooling device connected to the path of circulation.
At the time of starting up the reaction of catalytic gas phase oxidation in such a reactor, however, the reactor must be preheated, in advance of the supply of a raw material gas, in order to rise a temperature for promoting the reaction. Thus, a reactor of the so-called shell-and-tube type which is furnished as a heating means operated exclusively during the time of starting up with a heating device disposed additionally outside the reactor, connected to the reactor in the vicinity of the terminal part of the bundle of reaction tubes, and allowed to have a composite closing-adjusting mechanism installed at the site of connection has been disclosed in the official gazette of U.S. Pat. No. 3,762,465.
The heat medium which is used in the reactor of this sort is known in such types as organic heat medium, fused salt, and fused metal. Though the organic heat medium is widely used, it allows no safe use from the viewpoint of thermal stability at elevated temperatures exceeding 350° C. Generally, therefore, the fused salt (commonly called “niter”) is often used as a heat medium to be used at temperatures in the range of 350-550° C.
The composition of this niter embraces a mixture comprising 43% of sodium nitrite, 7% of sodium nitrate and 53% of potassium nitrate and a mixture consisting of 50% of sodium nitrite and 50% of potassium nitrate, for example. The solid point of the former mixture is 142° C., it is known that the niter has the solid point thereof rise when the mixing ratio of the components thereof varies and that the niter while in service suffers the solid point thereof to rise when the nitrous acid is converted into sodium nitrate by decomposition or oxidation. Generally, the reactor which uses the niter of this nature as a heat medium, therefore, is more often than not designed on the assumption that the solid point of this heat medium is 180° C.
When propylene, for example, is oxidized by using a catalyst for contact gas phase oxidation, the niter which is in a solid state at atmospheric temperature can maintain a fused state and attain easy circulation inside the reactor because the reaction temperature is higher than the solid point of the heat medium and the reaction itself is exothermal in nature. Since the reactor at the time of starting up is at a temperature lower than the solid point of the heat medium, however, the temperature of the reactor must be elevated till the heat medium in the reactor assumes a fused state.
FIG. 2 is a type section illustrating a large reaction apparatus for the production of phthalic anhydride, maleic anhydride, acrylic acid, and methacrylic acid, for example, and a route for the circulation of a heat medium therein. The flow of the heat medium at the time of starting up this reaction apparatus for the production of acrylic acid may be described as follows. With reference to FIG. 2, 101 stands for a reactor, 102 for an axial pump, 103 for a steam generator, 103′ for a boiler feed water, 103″ for steam, 104 and 106 each for a heater, 105 for a heat medium tank, and 107 for a pump. When the niter is used as the heat medium, since the niter is in a solid state at atmospheric temperature, the practice of discharging the niter from the reactor and putting it to storage in a heat medium tank after the use of the reactor is completed prevailing. The method for starting up the reactor in this case will be described.
First, the heat medium stored in the heat medium tank 105 is heated with the heater 104 passing steam therethrough till it is fused. The heat medium is supplied with the pump 107 to the reactor 101, circulated with the axial pump 102 to the fluid outside the tubes in the reactor, and thereafter heated with the electric heater 106 for elevation of the temperature thereof. The steam generator 103 is used for cooling the heat medium heated to an unduly high temperature or removing the heat of reaction after introduction of the raw material gas.
When the niter is used as the heat medium during the time of starting up the reactor, the reactor 101 itself may be heated for elevation of temperature with the electric heater 106 as illustrated in FIG. 2 or the heat medium heated with the electric heater for temperature elevation may be supplied to the reactor 101 as described above.
When the heat medium is circulated by the use of the heating device disposed simply outside the reactor as described in the official gazette of U.S. Pat. No. 3,762,465, the inside of the reaction tube is not heated till a fully satisfactory elevated temperature. Particularly during the time of starting up the reaction of catalytic gas phase oxidation, therefore, the circulation of the heat medium requires a long time and the shift of the reaction to the normal condition necessitates an unduly long time.
Further, since the heat medium has the density thereof vary with temperature, the total volume thereof is varied in accordance as the rate of reaction and the amount of the heat of reaction to be generated proportionately therewith are changed. Absolutely no measure, however, has been heretofore adopted for moderating the change of the volume proportionate with the change in the density of the heat medium. Rather the use of an apparatus capable of resisting pressure has been barely resorted to.
Particularly, when the compound at which the production is aimed happens to be acrylic acid, there are times when the reaction is performed in two stages, one for obtaining acrolein from propylene as the raw material and the other for subsequently obtaining acrylic acid from the acrolein. In this case, besides the procedure which comprises using a first reactor in the first reaction for the production of acrolein from propylene and a second reactor in the second reaction for the production of acrylic acid from acrolein, the procedure which, by the use of a reactor partitioned with an intermediate tube sheet into a first chamber and a second chamber, effects the first reaction in the first chamber and the second reaction in the second chamber may be adopted. In any event, in the process of production which combines a plurality of modes of reaction, as many reactors as the modes of reaction must severally attach heat medium heating devices and this annexation proves uneconomical from the viewpoint of equipment design and tends to complicate the work environment. In contrast, when the exothermal reaction is performed in the normal condition, since the heat medium is cooled outside the reactor and put to cyclic use, the heating devices which are not used unless at the time of starting up form an excess of equipment.